Battery charge indicator for implantable pacemakers and defibrillators

ABSTRACT

Current usage from a battery in an implantable cardiac device is tracked. The apparatus includes a battery current sensor having multiple current ranges. The current sensor produces a first signal representative of current drawn from a battery. A current range selector selects a current range for the battery current sensor and produces a second signal representative of the current range. An accumulator accumulates the first signal based on the second signal over time to generate an output signal representing usage of the battery.

FIELD OF THE INVENTION

The present invention relates generally to implantable cardiac devicesand, more particularly, to keeping track of battery usage in implantablecardiac devices

BACKGROUND

An implantable cardiac device is a medical device that is implanted in apatient to monitor electrical activity of the heart and to deliverappropriate electrical and/or drug therapy, as required. Implantablecardiac devices include, for example, pacemakers, cardioverters anddefibrillators. The term “implantable cardioverter defibrillator” orsimply “ICD” is used herein to refer to any implantable cardiac device.

An ICD employs a battery to power its internal circuitry and to generateelectrical therapy. The electrical therapy can include, for example,pacing pulses, cardioverting pulses and/or defibrillator pulses.

When a battery is manufactured, its energy capacity is known.Specifically, it is known how many Ampere-Hours of energy the batterycan deliver. Based on the known battery energy capacity and based onpredicted usage, battery life can be predicted and a replacementinterval established. With this approach, a conservative margin inremaining battery life is observed to prevent device failure due to adepleted battery. Alternatively, actual battery usage can be tracked,and the device can be replaced when the actual remaining energy capacityof the battery falls below a predetermined threshold. This secondapproach of tracking battery usage and replacing a device whenreplacement is actually required is preferred since it reducesunnecessary device replacements. It could, also, be used to inform thephysician of unexpected battery depletion or excessive current drainthat might be a sign of malfunction.

Conventional methods for tracking battery usage use estimationtechniques to determine how much energy is left in the battery. Aspreviously mentioned, the estimation techniques are not accurate andrequire observation of conservative margins. Other conventional systemsmeasure battery voltage and use the voltage measurement as an indicatorof how much energy is left in the battery. This method is problematicbecause of unexpected drops and hikes in voltage within the batterycircuit which can lead to inaccurate battery life predictions.

What is needed is a better and more accurate system and method forkeeping track of the battery usage, so that the end-of-life of thebattery can be predicted with greater precision.

SUMMARY

The present invention includes a method and apparatus for trackingcurrent usage over time from a battery in an implantable cardiac device.The apparatus includes a battery current sensor having multiple currentranges. The current sensor produces a first signal representative ofcurrent drawn from a battery. A current range selector selects a currentrange for the battery current sensor and produces a second signalrepresentative of the current range. An accumulator accumulates overtime the first signal based on the second signal to generate an outputsignal representing usage of the battery.

In a preferred embodiment, the current sensor and current range selectortogether form an automatic, multi-range current sensor. Current issensed as a voltage across a resistance placed in series with the loadcurrent being drawn from the battery. A higher resistance is used withlow currents and a lower resistance is used with higher currents. Thisyields a current sensor with good sensitivity and accuracy across a widecurrent range while keeping parasitic power loss to a minimum.

The multi-range current sensor classifies the drawn current into aselected one of four ranges. For example, a first current range is up toabout 128 μAmps, a second current range is up to about 4 mAmps, a thirdcurrent range is up to about 128 mA, and a fourth current range is up toabout 4 Amps. The range is selected by the current sensor as follows.

As indicated above, the current is sensed as a voltage across aresistance. An amplifier amplifies the voltage. A window comparatorcompares the magnitude of the voltage to two reference voltages. Basedon this comparison, the comparator produces up or down signals to acounter, causing the counter to increment or decrement. The output ofthe counter represents a range select signal. A decoder produces a rangecode from the range select signal. The current sensor uses the rangecode to select the resistance value placed in series with the batteryload current. The range select signal is then used by the accumulator asan indication of the range of the current measured.

The amplified voltage representing the load current is provided to ananalog to digital converter and converted to a digital value. Thedigital value, representing the load current, is then provided to theaccumulator. The accumulator uses the range select signal to determinethe weight to be given to the digital value representing the loadcurrent. In a preferred embodiment, the digital value representing theload current is an 8-bit number. The accumulator is a 40-bitaccumulator. Weighting of the digital value is done by selecting wherein the 40-bits, the 8-bit value is added. For example, for high currentvalues, the range select signal will cause the accumulator to add the8-bit value into more significant bits of the accumulator (e.g., bits 22through 15, with bit 39 being the most significant bit in theaccumulator). For low current values, the range select signal will causethe accumulator to add the 8-bit value into less significant bits of theaccumulator (e.g., bits 7 through 0, with bit 0 being the leastsignificant bit in the accumulator).

The method of the invention involves tracking battery usage in animplantable cardiac device. The method includes the steps of: (1)sensing a current being drawn from the battery, (2) generating a firstsignal representing the current; (3) classifying the current into aselected one of a plurality of predetermined ranges; (4) generating asecond signal indicative of the selected range; (5) using the secondsignal to accumulate the first signal over time; and (6) generating,based on the accumulation, a third signal representing usage of thebattery. The generating step includes: producing a voltage signalrepresenting the current, amplifying the voltage signal, and digitizingthe voltage signal to generate the first signal.

In a preferred embodiment, the classifying step includes classifying thecurrent into a selected one of four predetermined ranges: a firstcurrent range of up to about 128 μAmps, a second current range of up toabout 4 mAmps, a third current range of up to about 128 mA, and a fourthcurrent range of up to about 4 Amps.

The accumulation includes weighting the digitized voltage signal basedon the second signal and accumulating the digitized voltage signal overtime based on a clock signal.

Further features and advantages of the present invention as well as thestructure and operation of various embodiments of the present inventionare described in detail below with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the relevant art(s) to makeand use the invention. In the drawings, like reference numbers indicateidentical or functionally similar elements. Additionally, the leftmostdigit of a reference number identifies the drawing in which thereference number first appears.

FIG. 1A is a simplified diagram illustrating an ICD in electricalcommunication with at least three leads implanted into a patient's heartfor delivering multi-chamber stimulation and shock therapy.

FIG. 1B is a functional block diagram of an ICD which can providecardioversion, defibrillation and pacing stimulation in four chambers ofa heart.

FIG. 2A is a block diagram of a battery charge indicator circuit,according to the present invention.

FIG. 2B is a circuit diagram of a multirange current sensor within thebattery charge indicator circuit shown in FIG. 2A, according to thepresent invention.

FIG. 2C is a block diagram of a multi-range digital accumulator circuit,as shown in FIG. 2A, according to the present invention.

FIG. 3 is a flowchart illustrating a method for tracking battery usagein a cardiac device, according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the present invention refers tothe accompanying drawings that illustrate exemplary embodimentsconsistent with this invention. Other embodiments are possible, andmodifications may be made to the embodiments within the spirit and scopeof the present invention. Therefore, the following detailed descriptionis not meant to limit the invention. Rather, the scope of the inventionis defined by the appended claims.

It would be apparent to one of skill in the art that the presentinvention, as described below, may be implemented in many differentembodiments of hardware, software, firmware, and/or the entitiesillustrated in the figures. Any actual software and/or hardwaredescribed herein is not limiting of the present invention. Thus, theoperation and behavior of the present invention will be described withthe understanding that modifications and variations of the embodimentsare possible, given the level of detail presented herein.

Before describing the invention in detail, it is helpful to describe anexample environment in which the invention may be implemented. Thepresent invention is particularly useful in the environment of animplantable cardiac device. Implantable cardiac devices include, forexample, pacemakers, cardioverters and defibrillators. The term“implantable cardioverter defibrillator” or simply “ICD” is used hereinto refer to any implantable cardiac device or implantable cardioverterdefibrillator (“ICD”). FIGS. 1A and 1B illustrate such an environment.

As shown in FIG. 1A, there is an exemplary ICD 10 in electricalcommunication with a patient's heart 12 by way of three leads, 20, 24and 30, suitable for delivering multi-chamber stimulation and pacingtherapy. To sense atrial cardiac signals and to provide right atrialchamber stimulation therapy, ICD 10 is coupled to implantable rightatrial lead 20 having at least an atrial tip electrode 22, whichtypically is implanted in the patient's right atrial appendage.

To sense left atrial and ventricular cardiac signals and to provideleft-chamber pacing therapy, ICD 10 is coupled to “coronary sinus” lead24 designed for placement in the “coronary sinus region” via thecoronary sinus for positioning a distal electrode adjacent to the leftventricle and/or additional electrode(s) adjacent to the left atrium. Asused herein, the phrase “coronary sinus region” refers to thevasculature of the left ventricle, including any portion of the coronarysinus, great cardiac vein, left marginal vein, left posteriorventricular vein, middle cardiac vein, and/or small cardiac vein or anyother cardiac vein accessible by the coronary sinus.

Accordingly, exemplary coronary sinus lead 24 is designed to receiveatrial and ventricular cardiac signals and to deliver left ventricularpacing therapy using at least a left ventricular tip electrode 26, leftatrial pacing therapy using at least a left atrial ring electrode 27,and shocking therapy using at least a left atrial coil electrode 28.

ICD 10 is also shown in electrical communication with the patient'sheart 12 by way of an implantable right ventricular lead 30 having, inthis embodiment, a right ventricular tip electrode 32, a rightventricular ring electrode 34, a right ventricular (RV) coil electrode36, and an SVC coil electrode 38. Typically, right ventricular lead 30is transvenously inserted into heart 12 so as to place the rightventricular tip electrode 32 in the right ventricular apex so that RVcoil electrode 36 will be positioned in the right ventricle and SVC coilelectrode 38 will be positioned in the superior vena cava. Accordingly,right ventricular lead 30 is capable of receiving cardiac signals anddelivering stimulation in the form of pacing and shock therapy to theright ventricle.

FIG. 1B shows a simplified block diagram of ICD 10, which is capable oftreating both fast and slow arrhythmias with stimulation therapy,including cardioversion, defibrillation, and pacing stimulation. While aparticular multi-chamber device is shown, it is shown for illustrationpurposes only, and one of skill in the art could readily duplicate,eliminate or disable the appropriate circuitry in any desiredcombination to provide a device capable of treating the appropriatechamber(s) with the desired cardioversion, defibrillation and pacingstimulation.

A housing 40 of ICD 10, shown schematically in FIG. 1B, is oftenreferred to as the “can,” “case” or “case electrode” and may beprogrammably selected to act as the return electrode for all “unipolar”modes. Housing 40 may further be used as a return electrode alone or incombination with one or more of coil electrodes, 28, 36, and 38 forshocking purposes. Housing 40 further includes a connector (not shown)having a plurality of terminals, 42, 44, 46, 48, 52, 54, 56, and 58(shown schematically and, for convenience, the names of the electrodesto which they are connected are shown next to the terminals). As such,to achieve right atrial sensing and pacing, the connector includes atleast a right atrial tip terminal (AR TIP) 42 adapted for connection toatrial tip electrode 22.

To achieve left chamber sensing, pacing and shocking, the connectorincludes at least a left ventricular tip terminal (VL TIP) 44, a leftatrial ring terminal (AL RING) 46, and a left atrial shocking terminal(AL COIL) 48, which are adapted for connection to left ventricular ringelectrode 26, left atrial tip electrode 27, and left atrial coilelectrode 28, respectively.

To support right chamber sensing, pacing, and shocking the connectoralso includes a right ventricular tip terminal (VR TIP) 52, a rightventricular ring terminal (VR RING) 54, a right ventricular shockingterminal (RV COIL) 56, and an SVC shocking terminal (SVC COIL) 58, whichare configured for connection to right ventricular tip electrode 32,right ventricular ring electrode 34, RV coil electrode 36, and SVC coilelectrode 38, respectively.

At the core of ICD 10 is a programmable microcontroller 60 whichcontrols the various modes of stimulation therapy. As is well known inthe art, microcontroller 60 typically includes a microprocessor, orequivalent control circuitry, designed specifically for controlling thedelivery of stimulation therapy and can further include RAM or ROMmemory, logic and timing circuitry, state machine circuitry, and I/Ocircuitry. Typically, microcontroller 60 includes the ability to processor monitor input signals (data) as controlled by a program code storedin a designated block of memory. The details of the design ofmicrocontroller 60 are not critical to the present invention. Rather,any suitable microcontroller 60 can be used to carry out the functionsdescribed herein. The use of microprocessor-based control circuits forperforming timing and data analysis functions are well known in the art.In specific embodiment of the present invention, microcontroller 60performs some or all of the steps associated with tracking battery usagein accordance with the present invention.

Representative types of control circuitry that may be used with theinvention include the microprocessor-based control system of U.S. Pat.No. 4,940,052 (Mann et al.) and the state-machines of U.S. Pat. No.4,712,555 (Thornander et al.) and U.S. Pat. No. 4,944,298 (Sholder). Fora more detailed description of the various timing intervals used withinthe ICD's and their inter-relationship, see U.S. Pat. No. 4,788,980(Mann et al.). The '052, '555, '298 and '980 patents are incorporatedherein by reference.

As shown in FIG. 1B, an atrial pulse generator 70 and a ventricularpulse generator 72 generate pacing stimulation pulses for delivery byright atrial lead 20, right ventricular lead 30, and/or coronary sinuslead 24 via an electrode configuration switch 74. It is understood thatin order to provide stimulation therapy in each of the four chambers ofthe heart, atrial and ventricular pulse generators 70,72, may includededicated, independent pulse generators, multiplexed pulse generators,or shared pulse generators. Pulse generators 70 and 72 are controlled bymicrocontroller 60 via appropriate control signals 76 and 78,respectively, to trigger or inhibit the stimulation pulses.

Microcontroller 60 further includes timing control circuitry 79 which isused to control pacing parameters (e.g., the timing of stimulationpulses) as well as to keep track of the timing of refractory periods,PVARP intervals, noise detection windows, evoked response windows, alertintervals, marker channel timing, etc., which are well known in the art.Examples of pacing parameters include, but are not limited to,atrio-ventricular (AV) delay, interventricular (RV-LV) delay, atrialinterconduction (A—A) delay, ventricular interconduction (V—V) delay,and pacing rate.

Switch 74 includes a plurality of switches for connecting the desiredelectrodes to the appropriate I/O circuits, thereby providing completeelectrode programmability. Accordingly, switch 74, in response to acontrol signal 80 from microcontroller 60, determines the polarity ofthe stimulation pulses (e.g., unipolar, bipolar, combipolar, etc.) byselectively closing the appropriate combination of switches (not shown)as is known in the art.

Atrial sensing circuits 82 and ventricular sensing circuits 84 may alsobe selectively coupled to right atrial lead 20, coronary sinus lead 24,and right ventricular lead 30, through switch 74 for detecting thepresence of cardiac activity in each of the four chambers of the heart.Accordingly, the atrial (ATR. SENSE) and ventricular (VTR. SENSE)sensing circuits 82 and 84 may include dedicated sense amplifiers,multiplexed amplifiers, or shared amplifiers. Switch 74 determines the“sensing polarity” of the cardiac signal by selectively closing theappropriate switches, as is also known in the art. In this way, theclinician may program the sensing polarity independent of thestimulation polarity.

Each sensing circuit, 82 and 84, preferably employs one or more lowpower, precision amplifiers with programmable gain and/or automatic gaincontrol, bandpass filtering, and a threshold detection circuit, as knownin the art, to selectively sense the cardiac signal of interest. Theautomatic gain control enables ICD 10 to deal effectively with thedifficult problem of sensing the low amplitude signal characteristics ofatrial or ventricular fibrillation. Such sensing circuits, 82 and 84,can be used to determine cardiac performance values used in the presentinvention.

The outputs of atrial and ventricular sensing circuits 82 and 84 areconnected to microcontroller 60 which, in turn, are able to trigger orinhibit atrial and ventricular pulse generators, 70 and 72,respectively, in a demand fashion in response to the absence or presenceof cardiac activity, in the appropriate chambers of the heart. Sensingcircuits 82 and 84, in turn, receive control signals over signal lines86 and 88 from microcontroller 60 for purposes of measuring cardiacperformance at appropriate times, and for controlling the gain,threshold, polarization charge removal circuitry (not shown), and timingof any blocking circuitry (not shown) coupled to the inputs of sensingcircuits 82 and 86.

For arrhythmia detection, ICD 10 utilizes the atrial and ventricularsensing circuits 82 and 84 to sense cardiac signals to determine whethera rhythm is physiologic or pathologic. The timing intervals betweensensed events (e.g., P-waves, R-waves, and depolarization signalsassociated with fibrillation which are sometimes referred to as“F-waves” or “Fib-waves”) are then classified by microcontroller 60 bycomparing them to a predefined rate zone limit (i.e., bradycardia,normal, low rate VT, high rate VT, and fibrillation rate zones) andvarious other characteristics (e.g., sudden onset, stability,physiologic sensors, and morphology, etc.) in order to determine thetype of remedial therapy that is needed (e.g., bradycardia pacing,anti-tachycardia pacing, cardioversion shocks or defibrillation shocks,collectively referred to as “tiered therapy”).

Microcontroller 60 utilizes arrhythmia detection circuitry 75 andmorphology detection circuitry 76 to recognize and classify arrhythmiaso that appropriate therapy can be delivered.

Cardiac signals are also applied to the inputs of an analog-to-digital(A/D) data acquisition system 90. Data acquisition system 90 isconfigured to acquire intracardiac electrogram signals, convert the rawanalog data into a digital signal, and store the digital signals forlater processing and/or telemetric transmission to an external device102. Data acquisition system 90 is coupled to right atrial lead 20,coronary sinus lead 24, and right ventricular lead 30 through switch 74to sample cardiac signals across any pair of desired electrodes.

Advantageously, data acquisition system 90 can be coupled tomicrocontroller 60, or other detection circuitry, for detecting anevoked response from heart 12 in response to an applied stimulus,thereby aiding in the detection of “capture.” Capture occurs when anelectrical stimulus applied to the heart is of sufficient energy todepolarize the cardiac tissue, thereby causing the heart muscle tocontract. Microcontroller 60 detects a depolarization signal during awindow following a stimulation pulse, the presence of which indicatesthat capture has occurred. Microcontroller 60 enables capture detectionby triggering ventricular pulse generator 72 to generate a stimulationpulse, starting a capture detection window using timing controlcircuitry 79 within microcontroller 60, and enabling data acquisitionsystem 90 via control signal 92 to sample the cardiac signal that fallsin the capture detection window and, based on the amplitude, determinesif capture has occurred.

The implementation of capture detection circuitry and algorithms arewell known. See for example, U.S. Pat. No. 4,729,376 (Decote, Jr.); U.S.Pat. No. 4,708,142 (Decote, Jr.); U.S. Pat. No. 4,686,988 (Sholder);U.S. Pat. No. 4,969,467 (Callaghan et al.); and U.S. Pat. No. 5,350,410(Kleks et al.), which patents are hereby incorporated herein byreference. The type of capture detection system used is not critical tothe present invention.

Microcontroller 60 is further coupled to a memory 94 by a suitabledata/address bus 96, wherein the programmable operating parameters usedby microcontroller 60 are stored and modified, as required, in order tocustomize the operation of ICD 10 to suit the needs of a particularpatient. Such operating parameters define, for example, pacing pulseamplitude, pulse duration, electrode polarity, rate, sensitivity,automatic features, arrhythmia detection criteria, and the amplitude,waveshape and vector of each shocking pulse to be delivered to thepatient's heart 12 within each respective tier of therapy.

Advantageously, the operating parameters of ICD 10 may be non-invasivelyprogrammed into memory 94 through a telemetry circuit 100 in telemetriccommunication with external device 102, such as a programmer,transtelephonic transceiver, or a diagnostic system analyzer. Telemetrycircuit 100 is activated by microcontroller 60 by a control signal 106.Telemetry circuit 100 advantageously allows intracardiac electrogramsand status information relating to the operation of ICD 10 (as containedin microcontroller 60 or memory 94) to be sent to external device 102through an established communication link 104.

For examples of such devices, see U.S. Pat. No. 4,809,697, entitled“Interactive Programming and Diagnostic System for use with ImplantablePacemaker” (Causey, III et al.); U.S. Pat. No. 4,944,299, entitled “HighSpeed Digital Telemetry System for Implantable Device” (Silvian); andU.S. Pat. No. 6,275,734, entitled “Efficient Generation of SensingSignals in an Implantable Medical Device such as a Pacemaker or ICD”(McClure et al.), which patents are hereby incorporated herein byreference.

In the preferred embodiment, ICD 10 further includes a physiologicsensor 108, that can be used to detect changes in cardiac performance orchanges in the physiological condition of the heart. Accordingly,microcontroller 60 can respond by adjusting the various pacingparameters (such as rate, AV Delay, RV-LV Delay, V—V Delay, etc.) inaccordance with the embodiments of the present invention.Microcontroller 60 controls adjustments of pacing parameters by, forexample, controlling the stimulation pulses generated by the atrial andventricular pulse generators 70 and 72. While shown as being includedwithin ICD 10, it is to be understood that physiologic sensor 108 mayalso be external to ICD 10, yet still be implanted within or carried bythe patient. More specifically, sensor 108 can be located inside ICD 10,on the surface of ICD 10, in a header of ICD 10, or on a lead (which canbe placed inside or outside the bloodstream).

ICD 10 additionally includes a battery 110 which provides operatingpower to all of the circuits shown in FIG. 1B. For ICD 10, which employsshocking therapy, battery 110 must be capable of operating at lowcurrent drains for long periods of time, and then be capable ofproviding high-current pulses (for capacitor charging) when the patientrequires a shock pulse. Battery 110 must also have a predictabledischarge characteristic so that elective replacement time can bedetected. Accordingly, ICD 10 preferably employs lithium/silver vanadiumoxide batteries, as is true for most (if not all) current devices.

ICD 10 further includes a magnet detection circuitry (not shown),coupled to microcontroller 60. It is the purpose of the magnet detectioncircuitry to detect when a magnet is placed over ICD 10, which magnetmay be used by a clinician to perform various test functions of ICD 10and/or to signal microcontroller 60 that the external programmer 102 isin place to receive or transmit data to microcontroller 60 throughtelemetry circuit 100.

As further shown in FIG. 1B, ICD 10 is shown as having an impedancemeasuring circuit 112 which is enabled by microcontroller 60 via acontrol signal 114. The known uses for an impedance measuring circuit120 include, but are not limited to, lead impedance surveillance duringthe acute and chronic phases for proper lead positioning ordislodgement; detecting operable electrodes and automatically switchingto an operable pair if dislodgement occurs; measuring respiration orminute ventilation; measuring thoracic impedance for determining shockthresholds; detecting when the device has been implanted; measuringstroke volume; and detecting the opening of heart valves, etc. Theimpedance measuring circuit 120 is advantageously coupled to switch 74so that any desired electrode may be used. The impedance measuringcircuit 112 is not critical to the present invention and is shown onlyfor completeness.

In the case where ICD 10 is intended to operate as a cardioverter, paceror defibrillator, it must detect the occurrence of an arrhythmia andautomatically apply an appropriate electrical therapy to the heart aimedat terminating the detected arrhythmia. To this end, microcontroller 60further controls a shocking circuit 116 by way of a control signal 118.The shocking circuit 116 generates shocking pulses of low (up to 0.5Joules), moderate (0.5–10 Joules), or high energy (11 to 40 Joules), ascontrolled by microcontroller 60. Such shocking pulses are applied tothe patient's heart 12 through at least two shocking electrodes (e.g.,selected from left atrial coil electrode 28, RV coil electrode 36, andSVC coil electrode 38). As noted above, housing 40 may act as an activeelectrode in combination with RV electrode 36, or as part of a splitelectrical vector using SVC coil electrode 38 or left atrial coilelectrode 28 (i.e., using the RV electrode as a common electrode).

Cardioversion shocks are generally considered to be of low to moderateenergy level (so as to minimize pain felt by the patient), and/orsynchronized with an R-wave and/or pertaining to the treatment oftachycardia. Defibrillation shocks are generally of moderate to highenergy level (i.e., corresponding to thresholds in the range of 5–40Joules), delivered asynchronously (since R-waves may be too disorganizedto be recognize), and pertaining exclusively to the treatment offibrillation. Accordingly, microcontroller 60 is capable of controllingthe synchronous or asynchronous delivery of the shocking pulses.

In accordance with the present invention, ICD 10 further includes abattery charge indicator circuit 160. Battery charge indicator circuit160 monitors current drawn from battery 110 to improve prediction ofwhen battery 110 needs replacement. Battery charge indicator circuit 160is further described in FIGS. 2A–2C below. Also, FIG. 3 furtherillustrates a method for tracking battery current usage in ICD 10.

Consumption of current in ICD 10 happens in various current rangesdepending on the mode of operation. For example, in one embodiment, ICD10 can draw current anywhere in the range of 10 μA to 4 A. ICD 10 can bedrawing 3–4 A over a short period of time (e.g., to charge the highvoltage capacitors) and 10–20 μA over a long period of time (e.g.,during monitoring when no electrical stimuli are being delivered). Thepresent invention is able to accommodate tracking of different currentusage from the battery during these different modes of operation.

The battery current consumption is normally in the μA range forpacemakers with peaks occurring during the generation of the pacingpulses. For a cardioverter or defibrillator, the high voltage capacitorcharging uses battery current that is typically in the range of about 3A, and other functions of the battery circuit may draw 10 mA or morefrom the battery. The battery charge indicator circuit of the presentinvention is able to integrate all these different currents over thelifetime of the battery to accurately keep track of actual charge (i.e.,current multiplied by time or Ampere seconds (A-S)) drawn from thebattery. Because the low current drains occur over much longer periodsof time than the higher current drains, the low current drains typicallyaccount for a significant portion of the battery consumption.

FIG. 2A is a block diagram of battery charge indicator circuit 160according to the present invention. Battery charge indicator circuit 160includes a multirange current sensor 203, a signal conditioning andvariable gain amplifier (VGA) 207, an analog to digital converter (ADC)211 and a multi-range digital accumulator 215. A load current 202 ispassed from battery 110 through multirange current sensor 203.Multirange current sensor 203 produces a voltage signal 205representative of the magnitude of load current 202. Multirange currentsensor 203 also produces range select signals 221, 222 representing arange of the load current. Range select signals 221, 222 are provided toaccumulator 215.

VGA 207, together with the multirange current sensor 203, conditions andamplifies voltage signal 205 to be within an operating range of ADC 211.Voltage signal 205 becomes an amplified signal 209 that is digitized byADC 211. In this embodiment of the present invention, ADC 211 digitizesamplified signal 209 into an 8-bit digital signal 213. Bits of digitalsignals 213 are then accumulated by accumulator 215. In one embodiment,ADC 211 samples the signal 209, 128 times per second, i.e. it generatesan 8-bit output about every 7.8 ms.

Accumulator 215 generates an output signal 217 representing the amountof energy that has been drawn from battery 110. Output signal 217 isreceived by microcontroller 60 (not shown in FIG. 2A). In thisembodiment, the capacity of accumulator 215 is forty bits, therefore,output signal 217 is a forty-bit output signal. Practically, themicrocontroller 60 may utilize, for example, only the most significant 8or 16 bits.

Because ICD 10 draws various currents from battery 110, load current 202fluctuates between several different “current ranges.” Each currentrange corresponds to a specific charge amount or energy range that isbeing drawn from the battery 110. In this embodiment of the presentinvention, battery charge indicator circuit 160 is configured to operatein four “current ranges.” In other words, based on different magnitudesof load current 202, multirange current sensor 203 generates differentvoltage signals 205 that correspond to different current ranges. Becausethe voltage signal 205 is amplified and digitized, a lower load current202 generates a digital signal 213 corresponding to a lower “currentrange” and, thus, lower charge consumption from battery 110. Conversely,a higher load current 202 generates a digital signal 213 correspondingto a higher “current range” and, thus, higher charge consumption frombattery 110.

FIG. 2B is a circuit diagram showing multirange current sensor 203 andVGA 207. Multirange current sensor 203 includes resistors R1, R2, R3,and R4 connected in series with load current 202. Switches SW1–SW7 arecoupled to resistors R1–R4. Switches SW1–SW7, in various configurations(i.e., some switches closed and some open), route the current drawn frombattery 110 through various series combinations of resistors R1–R4.Switches SW1–SW7 are controlled through various logic operationsdescribed below. As would be understood by one having ordinary skill inthe art, other combinations of switches and resistors are possible.

Load current passing through various combinations of resistors R1–R4,will produce a voltage drop that is measured across the resistors. Thisvoltage drop is then measured as an indication of load current 202.Different resistances are chosen based on the magnitude of the current.For example, smaller resistances are placed in series with largercurrents, and larger resistances are placed in series with smallercurrents, so that the measured voltage drop can be controlled to bewithin a desired voltage range.

As described above, ICD 10 consumes energy in predetermined currentranges depending on its mode of operation. Each current rangecorresponds to a particular resistor (e.g., R1) or combination ofresistors (e.g., R1–R4) being selected for load current 202 to passthrough. This embodiment of the present invention has four currentranges. For example, ICD 10 can consume energy from battery 110 in afirst current range of up to about 128 μA, a second current range of upto about 4 mA, a third current range of up to about 128 mA, or a fourthcurrent range of up to about 4 A. The following Table 1 illustrates arelationship between each “current range” and corresponding resistorsR1–R4 being connected in series with load current 202.

TABLE 1 Relationship between “current ranges” and activation of R1–R4Resistors being Current Digital connected to load Current Range Codecurrent 202 Range 1 00 R1 & R2 & R3 & R4 ~128 μA 2 01 R1 & R2 & R3 ~4 mA3 10 R1 & R2 ~128 mA 4 11 R1 ~4 A

In Table 1, the “current range” column indicates in which current rangeICD 10 is drawing current from battery 110. The “digital code” columnindicates a digital code related to each current range. The digital codeis discussed in detail below. The “resistors being connected to loadcurrent 202” column indicates resistors or combination of resistorsR1–R4 connected in series with load current 202. The “current” columnindicates a current range within which ICD 10 is drawing current. Forexample, the second current range corresponds to resistors R1, R2, andR3 being connected in series with load current 202, and ICD 10 drawingcurrent in the range of up to 4 mA.

Resistors or combinations of resistors R1–R4 are connected in serieswith load current 202 using switches SW1–SW7. The following Table 2illustrates which switches or combinations of switches SW1–SW7 activatecorresponding resistors or groups of resistors R1–R4. The digital code,shown in Table 1, indicates a switch being closed, along withappropriate resistors R1–R4 being connected to load current 202.

TABLE 2 Resistor activation and corresponding switching scheme SwitchSW1 SW2 SW3 SW4 SW5 SW6 SW7 Code for 00 01 10 11 01 + 10 + 11 10 + 11 11which switch is closed where “+” denotes logical “OR”.

Digital code 00 indicates that switch SW1 is closed, and switchesSW2–SW7 are open. Since SW1 is closed, load current 202 flows throughresistors R1, R2, R3, and R4, as shown in FIG. 2B and Table 1. Thismeans that ICD 10 is drawing current from battery 110 in a first currentrange of up to about 128 μA.

Digital code 01 indicates that switches SW2 and SW5 are closed. As aresult, load current 202 flows through resistors R1, R2 and R3. ResistorR4 is bypassed. This means that ICD 10 is drawing current from battery110 in a second current range of up to about 4 mA.

Digital code 10 indicates that switches SW3, SW5 and SW6 are closed. Asa result, load current 202 flows through resistors R1 and R2. ResistorsR3 and R4 are bypassed. This means that ICD 10 is drawing current frombattery 110 in a third current range of up to about 128 mA.

Digital code 11 indicates that switches SW4, SW5, SW6 and SW7 areclosed. As a result, load current 202 flows through resistor R1.Resistors R2, R3 and R4 are bypassed. This means that ICD 10 is drawingcurrent from battery 110 in a fourth current range of up to about 4 A.

The digital codes control logic circuitry, within multirange currentsensor 203, that controls the configuration of switches SW1–SW7. Thedigital codes are generated as discussed below.

As shown in FIG. 2B, VGA 207 includes an operational amplifier 271 whichreceives voltage signal 205 (representing the voltage drop across theselected combination of resistors R1–R4) from. Amplifier 271 amplifiesvoltage signal 205 to produce amplified signal 209. As depicted in FIGS.2A and 2B, amplified signal 209 is provided by analog-to-digitalconverter 211 for conversion to a digital signal, and then toaccumulator 215 for accumulation. Amplified signal 209 is also providedback to multirange current sensor 203 for range selection.

Referring to FIG. 2B, multirange current sensor 203 further includes ananalog window comparator 272, a two-bit counter 278 and a two-bitdecoder 279. Analog window comparator 272 is coupled to receive theoutput of amplifier 271 and to generate an UP signal 274 or a DOWNsignal 276. Two-bit counter 278 is coupled to receive UP signal 274 andDOWN signal 276 from comparator 272 and produces a two-bit count valuethat is supplied to two-bit decoder 279. The two-bit count value alsoserves as range select signals 221, 222. The output of two-bit decoder279 is a two-bit code value 280.

Comparator 272 compares voltage signal 205 to threshold voltages REF1and REF2, REF1 being higher than REF2. Threshold voltages REF1 and REF2are set such that REF1/REF2 is greater than the non-overlapping ratiowhich, in the current invention, is 32 or 5 bits. This providesHysteresis when switching ranges and prevent system oscillation.Threshold voltages REF1 and REF2 define a voltage window. Based on thiscomparison, analog window comparator 272 generates either UP signal 274or DOWN signal 276. UP signal 274 and DOWN signal 276 indicate,respectively, whether voltage signal 209 is greater than or less thanrespective threshold voltages REF1 and REF2.

Normally, in any current range, the voltage signal 209 is betweenthreshold voltages REF1 and REF2 (i.e., the voltage signal is within thewindow), then both UP signal 274 and DOWN signal 276 will be a logicalLOW. This will cause counter 278 to maintain its value without counting.As a result, the range select signals and range code will not change.

If voltage signal 209 is greater than threshold voltage REF1, then UPsignal 274 will be a logical HIGH, and DOWN signal 276 will be a logicalLOW. This will cause counter 278 to count up. Counting up increases therange select signals 221, 222 and the range code 280. Since, the rangecode controls range selection by multirange current sensor 203, adifferent current range is selected by reconfiguring switches SW1–SW7.In this example, increasing range code 280 will cause the resistanceplaced in the path of the load current to be reduced to thereby reducevoltage signal 205.

Conversely, if voltage signal 209 is less than REF2, then DOWN signal276 will be a logical HIGH, and UP signal 274 will be a logical LOW.This will cause counter 278 to count down. Counting down decreases therange select signals 221, 222 and the range code 280 and results in theresistance placed in the path of the load current to be increased tothereby increase voltage signal 205.

The following Table 3 illustrates various comparisons between voltagesignal 209 and threshold voltages REF1 and REF2.

TABLE 3 Comparison between voltage signal 209 and threshold voltagesREF1 and REF2 Condition UP signal 274 DOWN signal 276 voltage signal209 > REF1 HIGH LOW voltage signal 209 between LOW LOW REF1 and REF2voltage signal 209 < REF2 LOW HIGH

Table 4 illustrates how range select signals 221 and 222 correspond todigital codes 280.

TABLE 4 Correlation between range select signals 221, 222 and digitalcodes 280 Digital Code Range select signal Range select signal 280 221222 11 1 1 10 1 0 01 0 1 00 0 0

As previously discussed, range select signals 221 and 222 indicate fromwhich current range, ICD 10 is drawing current from battery 110. Rangeselect signals 221 and 222 are supplied to accumulator 215. Range codes280 are used only internal to multirange current sensor 203 for rangeselection.

Because load current 202 is continuously supplied to multirange currentsensor 203, voltage signal 209 is continuously compared to thresholdvoltages REF1 and REF2. Therefore, digital codes 280, controllingswitches SW1–SW7, are continuously generated as necessary causingresistors or combination of resistors R1–R4 to be switched in and out ofseries connection with load current 202, as necessary. Overlap betweenthe current ranges provides some hysteresis to prevent systemoscillation between current ranges. This overlap also reduces theimportance of the accuracy of the window comparator that causes rangeselection when a current is near a range boundary.

In an example embodiment, resistor R1 has a value of 16 mΩ (0.016Ω),resistor, R2 has a value of 0.5Ω, resistor R3 has a value of 16Ω, andresistor R4 has a value of 500Ω. Given the maximum current that will bepassed through each resistor (or resistor combination), theseresistances will result in a maximum voltage of about 64 mV (voltagesignal 205). In addition, amplifier 207 will have a gain of about 40 toyield an output voltage of about 2.5 V maximum at the input of windowcomparator 272 (voltage signal 209). In this embodiment, REF1 is set atabout 2.1 V, and REF2 is set at about 50 mV.

Based on these example values, the thresholds at which multirangecurrent sensor 203 will change range are set forth below in Table 5.

TABLE 5 Example Current Range Parameters Lower Upper Range Resist. LSBThreshold Threshold 1 516.5 Ω 0.5 μA — 102 μA 2 16.5 Ω 16 μA 75.8 μA 3.2mA 3 0.5 Ω 0.5 mA 2.5 mA 105 mA 4 0.016 Ω 15.6 mA 78.1 mA —

Note that Table 5 corresponds to Table 1 set forth above. The resistancevalues listed are total resistances calculated using the resistor valuesset forth in the example embodiment discussed immediately above. Theleast significant bit (LSB) values are calculated by dividing thecurrent range value by 256 (since 8 bits are used to represent thecurrent). The lower threshold value is calculated by dividing the lowerthreshold voltage (e.g., 0.05V) by the product of the gain (e.g., 40) ofamplifier 207 and the total resistance. For example, for current range2, the equation is 0.05V/(40·16.5Ω)=75.8 μA. The upper threshold valueis calculated by dividing the upper threshold voltage (e.g., 2.1V) bythe product of the gain (e.g., 40) of amplifier 207 and the totalresistance. For example, for current range 2, the equation is2.1V/(40·16.5Ω)=3.2 mA.

As illustrated by Table 5, overlap between the current ranges providessome hysteresis. For example, multirange current sensor 203 will switchfrom range 1 to range 2, when the current reaches 102 μA but will notswitch back from range 2 to range 1 until the current falls below 75.8μA.

Referring again to FIG. 2A, ADC 211 receives amplified voltage signal209 and digitizes it. Amplified voltage signal 209 is converted into an8-bit digital signal 213. Digital signal 213 is then supplied toaccumulator 215. Digital signal 213, along with range select signals 221and 222, identifies a particular current value. The bits representingdigital signal 213 are accumulated by accumulator 215 as describedbelow.

In the preceding example and throughout the text, all values and rangesare provided as examples to illustrate the invention and selection ofappropriate components. For example, since ranges 2, 3 and 4 coverexclusively the most significant five bits of the respective range, theexact value of each of these ranges is 32 times the preceding range. Inthis embodiment, Range 1 is chosen to be 128 uA. Accordingly, Range 2should be 4.096 mA, and Range 3 would be 131.072 mA and so on. The sameis true for the resistor values that determine each range. In FIG. 2B,R1 (which is the Range 4 sense resistor) is chosen to be 0.016 Ohm.Consequently, the Range 3 sense resistor (which is R1+R2), according toTable 1, makes R2 equal to 32*0.016−0.016=0.496 Ohm. Values of the othersense resistors can be calculated in the same manner. It would beapparent to a person skilled in the art to choose, for example, a valuefor the Range 1 sense resistor and then calculate values for the othersense resisters accordingly. Similarly, a value for current Range 4 maybe arbitrarily selected and values for the other ranges selectedaccordingly. Other alternatives could also be implemented, e.g., numberof ranges, number of overlap bits and/or number of resolution bits.

FIG. 2C is a block diagram illustrating accumulator 215, according tothe present invention. Accumulator 215 includes a multi-range selector240, a forty-bit adder 255 and a forty-bit latch 250 clocked by a clocksignal 201. Multi-range selector 240 receives digital signal 213 fromADC 211. Multi-range selector 240 processes digital signal 213 based onrange select signals 221 and 222. As described above, range selectsignals 221 and 222 indicate the current range of digital signal 213.Multi-range selector 240 includes digital bit ranges 242 a–242 d whichcorrespond to current ranges one through four as set forth in Table 1above. The following Table 6 illustrates digital bit ranges 242 andcorresponding current ranges.

TABLE 6 Digital bit ranges and corresponding current ranges. CurrentRange Digital bit range ~128 μA bits 07–00 ~4 mA bits 12–05 ~128 mA bits17–10 ~4 A bits 22–15

Because, in this embodiment of the present invention, accumulator 215 isa forty-bit accumulator, each digital bit range 242 corresponds to acertain number of bits (e.g., 8 bits in this embodiment, because digitalsignal 213 is an 8-bit value). Table 6 illustrates which bit positionscorrespond to which current range. For example, the first current rangeis represented by bit positions 07–00, the second current range isrepresented by bit positions 12–05, the third current range isrepresented by bit positions 17–10, and the fourth current range isrepresented by bit positions 22–15. The remaining bits of theaccumulator are used for overflow as current values are accumulated.

When multi-range selector 240 receives digital signal 213 from ADC 211,it selects a particular digital bit range 242 based on range selectsignals 221,222. For example, if multi-range selector 240 receives rangeselect signals 221,222 of “11,” digital signal 213 is indicated ascorresponding to bit range 242 d (bit positions 22–15). In response tothis range select signal, multi-range selector 240 will produce a 23-bitword with the 8-bit current value shifted into the appropriate bitpositions.

The 23-bit word produced by multi-range selector 240 is passed on line247 to adder 255 at input 257. Adder 255 adds this 23-bit word to thecurrent accumulated value 249 received at input 259 from 40-bit latch250. Adder 255 then outputs the sum value 217 representing anaccumulation of all current drawn from battery 110 over time. This sumvalue 217 is also stored in 40-bit latch 250. In this manner, adder 255and latch 250 accumulate load current values received from selector 240.Prior to receipt by adder 255, each current value received from selector240 has been shifted to the appropriate significant bit position asindicated by range select signals 221,222 prior to receipt by adder 255.

Latch 250 is clocked by a 128 Hz clock signal 201. This clock signalcontrols the accumulation rate of accumulator 215.

The accumulated current value 217 is provided to microcontroller 60 andis used determine battery usage. In the embodiment depicted in FIG. 1B,battery charge indicator 160 is depicted as being part ofmicrocontroller 60. In an alternate embodiment, battery charge indicator160 is implemented with circuitry within ICD 10, but distinct frommicrocontroller 60.

The following pseudo code illustrates operation of accumulator 215:

// 40-bit Multi-Range Digital Accumulator circuit module mrda (adc,range_select, clock_128 hz, por_n, accum_out); output [39:0] accum_out;  // 40-bit output reg [39:0] accum_out; input [7:0] adc; // 8-bit ADCinput wire [7:0] adc; input [1:0] range_select; // 2-bit Range Selectwire [1:0] range_select; input clock_128 hz, // 128 Hz clock por_n;   // active low Power-On Reset wire clock_128 hz,  por_n; //internal nets reg [39:0] latch_out; // latch output wire [22:0]multrng_out; // multi-range selector output // Hardware Definitions //40 bit adder - The MSB 17 bits of A input are zero assign accum_out ={17'b00000000000000000, multrng_out[22:0]} + latch_out[39:0]; // 40-bitlatch always @( negedge por_n or posedge clock_128 hz)  if(!por_n) latch_out[39:0] <= 40'h0000000000;  else  latch_out[39:0] <=accum_out[39:0]; // multi-range selector // when range_select is 11, adcis output on bits 22-15 // when range_select is 10, adc is output onbits 17-10 // when range_select is 01, adc is output on bits 12-5//    otherwise, adc is output on bits 7-0 assign multrng_out[22:0] =   (range_select[1:0] == 2'b11)?      {adc[7:0], 15'b000000000000000}:   (range_select[1:0] == 2'b10)?      {5'b000000, adc[7:0],10'b0000000000}:    (range_select[1:0] == 2'b01)?      {10'b00000000000,adc[7:0], 5'b00000}:      {15'b0000000000000000, adc[7:0]}: endmodule

In this embodiment of the present invention, accumulator 215 has a40-bit capacity. This means that accumulator 215 is capable ofaccumulating 40 bits of digital data. However, if desired, the capacityof accumulator 215 can be increased. Increasing the number of bits inaccumulator 215 will allow accumulator 215 to track battery usage forlarger capacity batteries.

Referring, again, to FIGS. 2A and 2B, further description is providedconcerning operation of battery charge indicator 160. Becauseaccumulator 215 is clocked with clock signal 201, the accumulated bitsof digital signals 213 correspond to current-time values of load current202. In other words, each load current 202 has a “bit-weight” valuemeasured in Ampere-Seconds (A-S).

The following formula (1) defines a bit-weight value for each bit indigital signal 213:

$\begin{matrix}{{{Bit}\mspace{14mu}{{Weight}\left( {A - S} \right)}} = \frac{{Value}\mspace{14mu}{of}\mspace{14mu}{load}\mspace{14mu}{current}\mspace{14mu} 202\mspace{14mu}{bit}\mspace{14mu}{{weight}(A)}}{{Value}\mspace{14mu}{of}\mspace{14mu}{clock}\mspace{14mu}{signal}\mspace{14mu} 201({Hz})}} & (1)\end{matrix}$where value of clock signal 201 is 128 Hz and value of load current 202bit weight varies according to bit position. Table 7 illustratesbit-weight values according to formula (1) and bit positions inforty-bit output signal 217.

TABLE 7 Bit positions in the accumulator and corresponding bit-weightvalues Bit Bit Weight Position (A-S) 0 3.91 n 1 7.81 n 2 15.63 n 3 31.3n 4 62.5 n 5 125.0 n 6 250.0 n 7 0.5μ 8 1.0μ 9 2.0μ 10 4.0μ 11 8.0μ 1216.0μ 13 32.0μ 14 64.0μ 15 128.0μ 16 256.0μ 17 512.0μ 18 1.02 m 19 2.05m 20 4.10 m 21 8.19 m 22 16.38 m 23 32.77 m 24 65.54 m 25 131.1 m 26262.1 m 27 524.3 m 28 1.05 29 2.10 30 4.19 31 8.39 32 16.78 33 33.55 3467.11 35 134.22 36 268.44 37 536.87 38 1073.74 39 2147.48

As shown in Table 7, because accumulator 215 has a storage capacity offorty bits, there are forty bit positions (numbered 0 to 39). Each bitposition corresponds to a particular current being drawn from battery110. Based on Table 7, it can be seen that accumulator 215 has acapacity of about 4295 Amp-seconds.

FIG. 3 illustrates a method for tracking usage of battery 110 in an ICD10, according to the present invention. In step 301, multirange currentsensor 203 detects a current being drawn from battery 110. As describedabove, multirange current sensor 203 detects current being drawn frombattery 110 and generates a voltage signal 209 indicative of the currentbeing drawn.

In step 302, voltage signal 209 is amplified and digitized. Then,amplified and digitized voltage signal 209 is classified into one offour digital ranges corresponding to its respective current range. Theprocessing then proceeds to step 303. In step 303, multirange currentsensor 203 generates range select signals 221 and 222 indicative of thecurrent range at which current is drawn. Range select signals 221 and222 are used by accumulator 215 to determine the relative magnitude ofthe load current.

In step 304, accumulator 215 receives range select signals 221,222 andamplified and digitized voltage signal 209 (digital signal 213) from ADC211. As described above, accumulator 215 assigns a bit position for thedigital signal 213. Then, accumulator 215 accumulates bits of thedigital signal 213 based on range select signals 221,222. Accumulator215 then generates a 40-bit output signal 217 representing usage ofbattery 110, as shown in step 304.

Example embodiments of the methods, systems, and components of thepresent invention have been described herein. As noted elsewhere, theseexample embodiments have been described for illustrative purposes only,and are not limiting. Other embodiments are possible and are covered bythe invention. Such embodiments will be apparent to persons skilled inthe relevant art(s) based on the teachings contained herein. Thus, thebreadth and scope of the present invention should not be limited by anyof the above-described exemplary embodiments, but should be defined onlyin accordance with the following claims and their equivalents.

1. A method for tracking battery usage in an implantable cardiac device,the method comprising: routing current drawn from a battery through acurrent one of a plurality of selectable circuit configurations, eachconfiguration corresponding to one of a plurality of ranges of currentvalues; measuring the voltage drop across the current circuitconfiguration; generating a first signal based on the voltage drop, thefirst signal indicative of the load current being drawn from thebattery; for each first signal, generating a corresponding second signalindicative of the current range within which the load current falls;switching to another of the plurality of selectable circuitconfigurations if the load current falls outside the range of thecurrent circuit configuration; repeating the measuring, generating andswitching over time; and determining usage of the battery byaccumulating the load current over time as a function of correspondingfirst and second signals.
 2. An apparatus for tracking battery usage inan implantable cardiac device, said apparatus comprising: a componentnetwork coupled to receive current from the battery and have anassociated voltage drop indicative of the current received from thebattery, the network including a plurality of components and switchesfor arranging the components in one of a plurality of configurations,each configuration corresponding to a different range of current values;an amplifier coupled to the component network and operative to output anamplified signal indicative of the current received from the battery; acurrent range selector coupled to receive the amplified signal andoperative to output a range select signal indicative of the range ofcurrent values within which the current received from the battery falls,and a control signal for setting the switches of the component networkto form the component configuration corresponding to the range ofcurrent values indicated by the range select signal; and an accumulatorcoupled to receive the amplified signal and the range select signal andoperative to determine usage of the battery based on the amplifiedsignal and the range select signal.
 3. The apparatus of claim 2, whereinthe ranges of current values comprise at least two ranges.
 4. Theapparatus of claim 3 wherein a first current range is of up to about 128μAmps; a second current range is of up to about 4 mAmps; a third currentrange is of up to about 128 mA; and a fourth current range is of up toabout 4 Amps.
 5. The apparatus of claim 2, wherein the current rangeselector comprises: a comparator for comparing a magnitude of theamplified signal to at least one reference voltage; means fordetermining the range select signal based on the comparison.
 6. Theapparatus of claim 2, further comprising: an analog to digital converterfor receiving and digitizing the amplified signal prior to the amplifiedsignal being provided to the accumulator.
 7. The apparatus of claim 6,wherein the accumulator is a forty-bit multi-range digital accumulator.8. The apparatus of claim 2 wherein the current range selectorcomprises: a comparator coupled to the amplifier to receive theamplified signal and compare the amplified signal to two referencevoltages; and means for producing the range select signal based on theoutput of the comparator.
 9. The apparatus of claim 8, wherein the meansfor producing the range select signal comprises: a counter coupled tothe comparator and configured to count up or down based on thecomparison by the comparator, the counter producing the range selectsignal.
 10. The apparatus of claim 2 wherein the at least two rangespartially overlap.
 11. The apparatus of claim 2 wherein the componentnetwork comprises a plurality of resistors that may be arranged in aplurality of different series configurations.
 12. The apparatus of claim2 wherein the current range selector comprises a decoder coupled toreceive the range select signals and to output corresponding controlsignals as a function of the range select signals.
 13. A circuit fortracking battery usage in an implantable cardiac device, said circuitcomprising: means for routing current drawn from a battery through acurrent one of a plurality of selectable circuit configurations, eachconfiguration corresponding to one of a plurality of ranges of currentvalues; means for measuring the voltage drop across the current circuitconfiguration; means for generating a first signal based on the voltagedrop, wherein the first signal is indicative of the load current beingdrawn from the battery; means for generating, for each first signal, acorresponding second signal indicative of the current range within whichthe load current falls; means for switching to another of the pluralityof selectable circuit configurations if the load current falls outsidethe range of the current circuit configuration; and means fordetermining usage of the battery by accumulating the load current overtime as a function of corresponding first and second signals.